Microbial interactions are often predicated on metabolism: e.g. auxotrophs depend on nutrients made by other microbes. Many algae are vitamin auxotrophs, with several known species requiring exogenous vitamin B12 [1]. This vitamin must be obtained from bacteria, as only they can synthesise it. Laboratory experiments have demonstrated mutualistic interactions between bacteria and B12 -dependent algae. Populations of the bacterium Mesorhizobium loti (B12 producer, carbon requirer) and the green alga Lobomonas rostrata (B12 requirer, carbon producer) in co-culture stabilise to an algae/bacteria ratio of 1/30, independent of initial inoculum ratio [2].

Here we consider the interactions between microbial populations separated in space. Experiments on hard agarose indicate that mutualistic interactions exist at a distance. We present a mathematical model capturing the essence of these experiments: growing populations of algae and bacteria coupled by a diffusive channel. Solutions to the model reveal rich dynamics. We will discuss these and speculate on the ecological significance of our findings for understanding environmental biofilms and microbial mats.